1. Field of the Invention
The present invention relates to lithography. More specifically, the present invention relates to corrections in illumination uniformity of light sources in a lithography system.
2. Related Art
In a state-of-the-art projection lithography system, the exposure of light from the source optics is transmitted through, or reflected from, the patterned reticle and then imaged by projection optics onto the surface of a resist coated wafer. Modern lithography machines scan the wafer and the reticle synchronously to expose a full rectangular field. The reticle has an absorbing/reflecting layer, typically chrome, in the border area that surrounds the circuit pattern to prevent unwanted exposure of the wafer. Additionally, a state-of-the-art projection lithography system usually has movable reticle-masking blades that: (1) remove the need for having a wide (i.e., expensive) border area, (2) block light that might otherwise leak through pin holes in the border area, (3) allow a selected portion of the full patterned area to be exposed, and (4) selectively block reticle alignment targets so that they are not printed on the wafer.
Current uniformity correction modules are calibrated separately for each pupil shape within the lithography system. This process is prohibitively lengthy. For example, current estimates for the time required to calibrate finger positions within a uniformity correction module are about 5-10 minutes per pupil shape. During a particular calibration sequence, a lithography tool will store calibration data for more than 100 pupil shapes, which is the number of pupil shapes that might be required to perform a reasonably thorough calibration. Thus, based on these parameters, calibration of 100 pupil shapes could require 8-17 hours.
Instability within an illuminator portion of the lithography system, such as transmission hysteresis, contamination, beam wander, etc. on the order of between 1 hour and 1 year, degrades the quality of calibrations performed by the uniformity correction module. This instability creates the need for frequent recalibrations. Recalibrations, especially across multiple pupil shapes, reduce lithography system productivity. Contamination within the optical system portion of the lithography system, for example, could cause the illumination uniformity specification to be exceeded every 4 months.
Current uniformity sampling plans are either too coarse for the available correction mechanism or too dense for the time allowed for calibration. Coarse sampling reduces calibration accuracy. Dense sampling reduces system productivity.
Calibration software for the uniformity correction module exists but has become very complex. Further, execution times of the finger positioning techniques have become prohibitive. Also, data storage has become another significant factor related to the complexity of the calibration software.
What is needed, therefore, is a calibration method and device that will overcome the deficiencies in conventional systems. More specifically, what is needed is a calibration system that facilitates profile measurement during every exposure field. What is also needed is a uniformity control technique with intensity-based feedback to use in commanding moves of uniformity correction module finger positions to facilitate fine-tuning the finger positions.